1. Field of the Invention
The present invention relates to a loop brush for use in an electrophotographic image forming process, a lubricant application mechanism using the loop brush, and an image forming apparatus using the lubricant application mechanism.
2. Description of the Related Art
Improvement of image quality such as high resolution and photographic reproducibility has been demanded in order to provide the quality comparable to that of offset printing, using an electrophotographic image forming apparatus. Reduction in toner particle size and spheronization of toner are known as one of effective means for satisfying the demand for improvement of image quality.
On the other hand, with spheronization and reduction in particle size, toner easily slips through between an image carrier and a cleaning blade for scraping off residual toner. This is because the reduction in toner particle size increases adhesion due to Van der Waals force between toner and the image carrier, and toner reduced in size more easily intrudes between the cleaning blade and the image carrier (nip section). In addition, with spheronization of toner, toner easily rolls on between the cleaning blade and the image carrier and easily intrudes into the nip section, accordingly.
If toner slips between the cleaning blade and the image carrier, that is, if poor cleaning occurs, the residual toner is transferred onto an image during the next process to form black streak-like image noise, or the residual toner blocks light during the exposure step to prevent formation of a latent image, resulting in image noise. Such poor cleaning is an event to avoid.
Then, a technique for facilitating cleaning of spheronized toner or toner reduced in particle size is proposed, in which a material for reducing a friction coefficient of an image carrier is supplied onto the image carrier (for example, Japanese Laid-Open Patent Publication No. 2007-047514). As an example of this technique, it is known to apply a lubricant made of fatty acid metallic salt such as zinc stearate onto a surface of the image carrier using a brush. The application of a lubricant on the image carrier reduces the adhesion and friction force of toner on the image carrier, and therefore, toner can be scraped off sufficiently even only with a cleaning blade.
As a method of supplying a lubricant to an image carrier, it is known to externally add fatty acid metallic salt (for example, zinc stearate) in a powder state to toner and supply the external additive as well as toner to the image carrier through development to form a thin film with a cleaning blade. This lubricant development supply method does not require a special structure for supplying a lubricant and is thus very effective in terms of cost and space. Therefore, low-speed/mid-speed image forming apparatuses often adopt this method.
However, in the lubricant development supply method described above, the lubricant, which is externally added, is often charged by friction with toner while being agitated in the development device, so that the supply of lubricant becomes uneven. More specifically, when being charged to the same polarity as toner, the lubricant is supplied to an image section on the image carrier, whereas when being charged to the opposite polarity to toner, the lubricant is supplied to a non-image section on the image carrier. Therefore, when the same image is successively printed for a long time, the supply of lubricant becomes uneven to cause variations of the friction coefficient.
When low-density images continue for a long time, the supply of lubricant to the cleaning blade reduces per se, and, as a result, the friction coefficient is not reduced enough. On the other hand, if the amount of lubricant externally added is increased, the lubricant easily transfers to the carrier in the development device. If the lubricant transfers, the charged property of toner is lost, causing oppositely charged toner. The oppositely charged toner causes a phenomenon called “fog noise” in which toner is developed in the non-image section. Furthermore, the lubricant is applied to the development roller to reduce the friction coefficient and prevent proper conveyance of developer. This will cause inconvenience such as reduced image density in high-density images.
As another method of supplying a lubricant to an image carrier, a solid lubricant obtained by solidifying fatty acid metallic salt (for example, zinc stearate) is pressed against a rotatable brush, and the lubricant scraped off with the brush is applied to an image carrier (lubricant application method).
This lubricant application method requires, in addition to a cleaning blade, a solid lubricant and a roll-like brush for scraping off the solid lubricant to be supplied to an image carrier as well as a component for holding them and a spring for pressing. Thus, the cost is increased, and more space is required. However, the stability of applying a lubricant is relatively high because the lubricant is actively applied to the image carrier. Another advantage is that environmental dependency is reduced.
A straight-bristle brush is generally used as the brush for use in the lubricant application method. The straight-bristle brush is like a toothbrush, in which fiber tip ends are present on the surface of the brush. Because of the fibers being independently present, the brush surface can be kept in a relatively dense state and can be brought into abutment with the image carrier uniformly. While keeping a uniform abutment state with the image carrier, the straight-bristle brush has the following problem in grinding the lubricant.
The solid lubricant having a soft surface is shaved and consumed through grinding. In the initial state, the solid lubricant is shaved relatively evenly. However, with consumption, depressions and projections are gradually formed on the surface. When depressions and projections are formed, a plurality of fibers are concentrated on the depressions thereby to accelerate grinding in the depressions. On the other hand, the fibers do not come into abutment with the projections. Therefore, as the consumption proceeds, the depressions and projections of the lubricant become noticeable. As the consumption further proceeds, the depressions cannot be ground because of reduction in pressing force against the lubricant. As a result, the amount of supply of the lubricant to the image carrier is significantly reduced, and the lubricant does not function any longer.
In order to prevent this state, it is effective to increase the fiber density of the brush. However, when the fiber density is increased, toner accumulates in the brush thereby reducing the capability of scraping off the lubricant. Moreover, the brushing force against the image carrier increases, which gives damages such as streaky scratches on the image carrier and increasing wear.
A loop brush may be used instead of a straight-bristle brush in the lubricant application method as described above (for example, Japanese Laid-Open Patent Publication No. 2010-117523). The loop brush, formed like terry cloth, is formed by making bundles of fibers in a loop shape. With the use of such a loop brush, the lubricant can be ground evenly.
The reason for this is as follows. In the loop brush, unlike the straight-bristle brush, the fibers are formed in bundles and the rigidity of the fiber bundles is thus high. Therefore, the fiber bundles can grind the lubricant without being affected by some depressions and protrusions of the lubricant. Another factor may be that the fibers are less concentrated on the depressions because the tip ends of the loop brush are formed into loops.
With the use of the loop brush, the lubricant is evenly ground. Accordingly, the supply to the image carrier is not abruptly reduced even when the consumption proceeds. The lubricant can be thoroughly used, and therefore, even a relatively small lubricant is highly durable.
A method of producing a loop brush will be described briefly. First, a bundle of several fibers is repeatedly passed upward and downward through a long ribbon-like cloth (hereinafter also referred to as “base cloth”) having warp and weft woven in a mesh structure. The fiber bundle is protruded longer on one side of the base cloth to form a loop shape. Then, the long ribbon having loops is wound around a metal shaft of a brush at a prescribed angle, which is uniquely determined according to the ribbon width and the shaft diameter, to form a roll shape.
As described above, the loop brush is formed by bundling a plurality of fibers, so that the fibers do not spread on the brush surface as in the straight-bristle brush and the fiber bundles are regularly distributed. Therefore, the fiber bundles may be lined up in the circumferential direction of the brush, depending on the angle of winding the ribbon. In other words, the fiber bundles may not be present in some places in the circumferential direction of the brush.
If the fiber density of the brush is reduced, the distance (arrangement line spacing) between an arrangement line of fiber bundles (a straight line connecting adjacent fiber bundles) and an arrangement line adjacent thereto is increased. Then, even when the arrangement line is at an angle with the circumferential direction, the arrangement line may not intersect the adjacent arrangement line in one rotation of the brush. In such a state, a portion not in contact with the fiber bundles of the brush periodically appears in the longitudinal direction of the image carrier (irregular abutment occurs). As a result, the supply of the lubricant to the image carrier becomes uneven, which appears as streak noise mainly in a halftone image.
On the other hand, the uneven abutment can be effectively prevented by increasing the fiber density of the brush. However, in this case, toner may be deposited in the brush to prevent the lubricant from being scraped off, as is the case with the use of the straight-bristle brush as described above.